Equal tee, means the branch diameter of this tee is same with the main pipe (Run pipe) diameter of this tee. When we see only one size of tee in its description, it means this tee is the equal tee. for example, 6″ Tee mean the tee main pipe and branch pipe both are 6 inches. Sometimes the equal tee also described 6″X 6″ which is the same meaning of 4″ tee.
A Lateral pipe tee means a pipe fitting which is similar to a tee but has a side opening outlet branch at a 45 degree angle. It is a kind of steel pipe tee, but the branch direction is different with the straight tee . On the contrary to the lateral tee, the straight tee has a branch pipe direction of 90 degree to the run pipe.
The difference between equal tee and lateral tee:
When the usage of the 90 degree tees in the pipe system, the fluid go through the tee will bear a littlebigger resistance. So there may have turbulence flow resistance. It will wastemore energy. The Type Y tee let the angle of branch pipe and main pipe be a acute angle. This design angle normallyis 30 to 60 degrees. So this kind of tee, not only reduced the resistance alsonot allow the turbulence flow happen. It is stable pipe fittings and savingmore energy.
Source: Yaang Pipe Industry Co., Limited (www.yaang.com)